1. Technical Field
The present invention pertains to fume extraction systems for furnaces, in particular, electric arc furnaces for melting metals.
2. Discussion of the Related Art
Fume extraction systems are typically utilized in combination with electric arc furnace (EAF) in metals melting and refining installations to capture airborne particulate emissions and to exhaust certain flammable and hazardous gases that evolve during operation of the furnace systems. In particular, gases such as carbon monoxide (CO) and hydrogen (H2) are generated during the melt and refine process and must be properly vented and treated by the fume extraction system to ensure combustion of these gases occurs safely and within a temperature controlled and contained environment. Further, volatile organic compounds (VOC""s) may also be generated during the melt process and must also be properly treated to prevent their emission from the ventilation stack to the atmosphere.
A negative pressure or suction is generated within the fume extraction system, via an induced draft (ID) fan, to draw fumes including the previously noted gases and other particulate emissions (e.g., slag or dust) from the EAF into the fume extraction system for treatment therein. The ID fan pulls dust-laden fumes through a bag-house including filters, and then exhausts the filtered gases though a stack and into the atmosphere. Since the fumes exit the EAF at temperatures of up to about 3500xc2x0 F., the fumes are typically cooled prior to entering the bag-house to temperatures of 200xc2x0 F. or less using water-cooled ductwork. Dilution air is incorporated to provide cooling of these gases before contacting the bag filters.
A typical EAF melt shop employs a fume extraction system with a number of conduit branches to draw and remove fumes from the EAF and other locations. For example, a conventional fume extraction system may include a conduit branch to collect fumes from one or more ladle metallurgy furnace stations (LMF""s), which contribute a small portion of dust emissions to be processed by the fume extraction system, a second conduit branch to suction fumes through a hood or canopy disposed directly above the EAF, and a third branch to suction fumes directly from the xe2x80x9cfourth holexe2x80x9d exhaust duct located at the EAF roof. The fourth hole exhaust duct is so named because the roof of an alternating current EAF typically includes three holes for the arc electrodes to extend into the EAF and a xe2x80x9cfourth holexe2x80x9d facilitating removal of exhaust gases that evolve during melting of metal within the EAF. The fourth hole exhaust duct is water cooled for much of its length, or at least to lengths where the exhaust gas is expected to exceed about 1200xc2x0 F.
An air gap is provided in the fourth hole exhaust duct at a location proximate the EAF roof to allow for furnace tilting during tapping of the EAF as well as EAF roof movement to permit opening and charging of the EAF. Air is drawn into this gap by the ID fan during system operation to provide sufficient oxygen within the EAF ventilation duct for burning of combustible gases exiting the EAF. There can be significant concentrations of combustible gases (e.g., as much as 75% on a dry basis), such as CO and H2, in the EAF exhaust. These combustible gases must be safely burned in the downstream water-cooled EAF duct section so as to prevent explosions during system operation and to eliminate or reduce emissions of these species. Accordingly, two main objectives of an EAF fume extraction system are to collect dust and other particulate matter from the fumes in the filters of the bag-house and to safely burn combustible gases emerging from the EAF before these gases enter the bag-house. If the system is not operating properly, fugitive dust emissions can escape the melt shop which could violate air emissions regulations and cause uncomfortable or unsafe working conditions.
Operation of the fume extraction system is controlled with the use of dampers disposed at suitable locations along the canopy, EAF exhaust and LMF exhaust duct sections to modulate suction by these three duct sections. In particular, when the EAF roof is moved to open the EAF during charging (i.e., adding scrap metal to the EAF) or tapping (i.e., removing molten metal from the EAF), the EAF exhaust damper is typically closed or only partially open, and the canopy damper is fully opened to evacuate large bursts of fumes that may be generated (e.g., when dropping a charge bucket into the EAF). During the next batch melt cycle after tapping of the EAF, the canopy damper is typically set to a fixed position, and the EAF exhaust damper is also set to a fixed position or adjusted manually during system operation based upon visual observation of fumes escaping from the furnace roof.
Many EAF shops presently provide little or no automated mechanism for controlling damper operation, and thus the modulation of suction to the branch sections of the fume extraction system, during a batch melt process. Attempts have been made to automate control in a closed loop manner of the EAF duct suction damper by measuring a negative pressure in the furnace or duct with a static pressure tap mounted in the EAF roof, shell, or water-cooled duct section located downstream from the EAF. In essence, the idea is to adjust the EAF exhaust damper so as to continually maintain a certain level of negative pressure in the furnace shell, or in the immediate downstream water-cooled duct section. However, these attempts are rarely effective in practice, because the pressure taps easily become clogged or burn up and thus are not reliable. Even when such pressure control automation does function, it may not provide optimal system operation from an energy efficiency standpoint, as there can be periods during the melt process when too much air is being drawn through the furnace. The alternative is manual adjustment of the EAF exhaust damper, as noted above, in which the operator will set the damper such that a fixed amount of suction will be applied to the EAF throughout a batch melting process. Typically, the operator will open the EAF exhaust damper to make sure that little or no fumes escape the furnace and create a xe2x80x9cpuffingxe2x80x9d effect. This manual adjustment based upon the operator""s visual observations of the EAF can lead to reduced energy efficiency (i.e., increased KWH/ton), with a tendency of the operator to err on the side of sucking too hard to minimize xe2x80x9cpuffingxe2x80x9d so as to keep a clean indoor shop environment.
In addition, some EAF shops will also include a variable gap adjustment mechanism at the fourth hole exhaust air gap to modulate the amount of combustion air sucked into this gap. The gap adjustment mechanism includes a sliding, water-cooled sleeve to selectively close portions of the air gap. However, these variable gap systems are bulky and cumbersome, and the sliding sleeve will frequently be rendered inoperative due to the accumulation of slag or debris at the sleeve to limit or prevent its sliding movement.
Thus, there exists a need to provide an improved fume extraction system that is efficient and reliable in extracting fumes and dust from the EAF and ensuring sufficient combustion of gases during a batch melting process, while at the same time minimizing the amount of infiltration air drawn through the EAF at any given time.
Accordingly, it is an object of the present invention to provide a fume extraction system that safely processes and vents exhaust gases generated in a furnace to the environment.
It is another object of the present invention to control the amount of air drawn into the fume extraction system to achieve sufficient combustion of exhaust gases flowing within the system.
It is a further object of the present invention to automate the control of air drawn into the fume extraction system to optimize system performance and efficiency.
The aforesaid objects are achieved individually and/or in combination, and it is not intended that the present invention be construed as requiring two or more of the objects to be combined unless expressly required by the claims attached hereto.
In accordance with the present invention, a fume extraction system is provided including a combustion zone with an inlet that is connectable with an exhaust outlet of a furnace. The combustion zone receives an exhaust gas stream emerging from the furnace outlet during system operation, where the exhaust gas stream includes explosive gases that undergo combustion reactions within the combustion zone. The fume extraction system further includes a duct section aligned downstream from the combustion zone, and a suction unit arranged within the system to establish a negative pressure within the furnace, the combustion zone, and the duct section so as to draw the exhaust gas stream from the furnace outlet and through the combustion zone and duct section during system operation. An adjustable exhaust damper is disposed at a selected location between the inlet of the combustion zone and the suction unit.
A control system is also included that selectively controls the negative pressure applied to the furnace, the combustion zone and the duct section. In particular, the control system includes a gas sensor device disposed at a selected location within the system to measure a concentration of at least one of oxygen, carbon monoxide, hydrogen, carbon dioxide, water vapor and nitrogen within the exhaust gas stream, and a controller in communication with the gas sensor device and the exhaust damper. The controller effects opening and closing of the exhaust damper to selectively modify the negative pressure within the furnace, the combustion zone and the duct section based upon gas concentration measurements received from the gas sensor device. Utilizing this system, a negative pressure can be applied during system operation that is energy efficient and establishes optimal amounts of airflow through the fume extraction system to effectively combust the explosive gases which are exhausted from the furnace. In effect, the system operates to strike a balance by drawing in enough combustion air to ensure safe combustion of all combustible species, while avoiding the drawing of excess EAF infiltration air which reduces furnace electrical energy efficiency.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of specific embodiments thereof, particularly when taken in conjunction with the accompanying drawings wherein like reference numerals in the various figures are utilized to designate like components.